Multilevel Modulation for Cognitive Multiaccess Relay Channel

In this paper, we propose a simple protocol for the cognitive multiaccess relay channel (MARC) with no transmit channel-state information and analyze the resulting stable throughputs of the primary and cognitive nodes. We propose allowing the cognitive node (common relay in the MARC topology) to simultaneously relay data for both primary nodes and transmit its own data (cognitive data) using a multilevel modulation (MLM) scheme. This modulation scheme provides unequal protection for the primary and cognitive data, which increases the throughput of the primary nodes by relaying their messages, in turn increasing the transmission opportunities of the cognitive nodes. The proposed scheme is studied from a networking perspective, and the stable throughput for the primary and secondary nodes as a function of the modulation levels and channel statistics is derived using queueing theory.